It’s a design issue.
The main functionality of RMG is "holding, mixing and granulating of standard volume”. The bulk density is product specific and may not be same for all products. You product may be cotton (lowest density) or osmium (highest density), you can process only the standard volume in your RMG. Hence its capacity is mentioned in liters.
But, in case of FBD, its functionality is not only holding of a product, but also fluidization (lifting up) of its weight. The air inlet duct (of FBD) and exhaust ducts are designed based on the weight it should lift up. Hence, FBD capacity should be mentioned in kg only